1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and apparatuses useable in drilling installations for determining a wellbore influx condition using qualitative indications.
2. Discussion of the Background
During drilling operations, gas, oil or other well fluids at a high pressure may flow from the drilled formations into the wellbore created during the drilling process. An unplanned influx from the formation into the wellbore is referred to in the industry as a “kick” and may occur at unpredictable moments. If the fluid influx is not promptly controlled, the well, the equipment in the well, and the drilling vessel is at risk. In order to protect the well and/or the equipment at risk, an assembly of valves called blow-out preventers, or BOPs, are located and actuated to contain the fluids in the wellbore upon detection of such events or indications of imminence of such events.
A traditional offshore oil and gas drilling configuration 10, as illustrated in FIG. 1, includes a platform 20 (or any other type of vessel at the water surface) connected via a riser 30 to a wellhead 40 on the seabed 50. It is noted that the elements illustrated in FIG. 1 are not drawn to scale and no dimensions should be inferred from relative sizes and distances illustrated in FIG. 1.
Inside the riser 30, as illustrated in the cross-section view A-A′, there is a drill string 32 at the end of which a drill bit (not shown) may be rotated to extend the subsea well through layers below the seabed 50. Mud is circulated from a mud tank (not shown) on the drilling platform 20 inside the drill string 32 to the drill bit, and returned to the drilling platform 20 through an annular space 34 between the drill string 32 and a casing 36 of the riser 30. The mud maintains a hydrostatic pressure to counter-balancing the pressure of fluids in the formation being drilled and cools the drill bit while also transporting the cuttings generated in the drilling process to the surface. At the surface, the mud returning from the well is filtered to remove the cuttings, and re-circulated.
A blowout preventer (BOP) stack 60 is located close to the seabed 50. The BOP stack may include a lower BOP stack 62 attached to the wellhead 40, and a Lower Marine Riser Package (“LMRP”) 64, which is attached to a distal end of the riser 30. During drilling, the lower BOP stack 62 and the LMRP 64 are connected.
A plurality of blowout preventers (BOPs) 66 located in the lower BOP stack 62 or in the LMRP 64 are in an open state during normal operation, but may be closed (i.e., switched in a close state) to interrupt a fluid flow through the riser 30 when a “kick” event occurs. Electrical cables and/or hydraulic lines 70 transport control signals from the drilling platform 20 to a controller 80 that is located on the BOP stack 60. The controller 80 controls the BOPs 66 to be in the open state or in the close state, according to signals received from the platform 20 via the electrical cables and/or hydraulic lines 70. The controller 80 also acquires and sends to the platform 20, information related to the current state (open or closed) of the BOPs. The term “controller” used here covers the well known configuration with two redundant pods.
Traditionally, as described, for example, in U.S. Pat. Nos. 7,395,878, 7,562,723, and 7,650,950 (the entire contents of which are incorporated by reference herein), a mud flow output from the well is measured at the surface of the water. The mud flow and/or density input into the well may be adjusted to maintain a pressure at the bottom of the well within a targeted range or around a desired value, or to compensate for kicks and fluid losses.
The volume and complexity of conventional equipment employed in the mud flow control are a challenge in particular due to the reduced space on a platform of an offshore oil and gas installation.
Another problem with the existing methods and devices is the relative long time (e.g., tens of minutes) between a moment when a disturbance of the mud flow occurs at the bottom of the well and when a change of the mud flow is measured at the surface. Even if information indicating a potential disturbance of the mud flow is received from the controller 80 faster, a relatively long time passes between when an input mud flow is changed and when this change has a counter-balancing impact at the bottom of the well.
Operators of oil and gas installations try to maintain an equivalent circulating density (ECD) at the bottom of a well close to a set value. The ECD is a parameter incorporating both the static pressure and the dynamic pressure. The static pressure depends on the weight of the fluid column above the measurement point, and, thus, of the density of the mud therein. The density of the mud input into the well via the drill string 32 may be altered by crushed rock or by fluid and gas emerging from the well. The dynamic pressure depends on the flow of fluid. Control of the mud flow may compensate for the variation of mud density due to these causes. U.S. Pat. No. 7,270,185 (the entire content of which is incorporated by reference herein) discloses methods and apparatuses operating on the return mud path, below the water surface, to partially divert or discharge the mud returning to the surface when the ECD departs from a set value.
U.S. patent application Ser. No. 13/050,164 proposes a solution of these problems in which a parameter proportional with a mud flow emerging from the wellbore is measured and used for controlling the outflow. However, accurately assessing the emerging mud flow is a challenge in itself because, unlike the mud pumped into the well, the emerging mud may not have a uniform composition. The emerging mud may sometimes (not always) contain formation cuttings or gas. This lack of uniformity in the mud composition affects the density or a mass balance. Additionally the drill string may be moving eccentrically inside the casing affecting measurement of the parameter proportional with the emerging mud flow. The mud may not be conductive enough to use magnetic parameters. Accurate ultrasonic parameter measurement may be impeded by mud's viscosity.
Accordingly, it would be desirable to provide methods and devices useable in offshore drilling installations near the actual wellhead for early detection of kick events or detecting indications of an imminence of a kick event, thereby overcoming the afore-described problems and drawbacks.